Antimicrobial agents from Mycoplasma species are being characterized to determine whether they differ from antimicrobial agents previously mentioned in the literature. Agents from a tetracycline- resistant strain of Acholeplasma laidlawii B appear to differ from previously described antibiotics in that a strain resistant (laboratory induced) to the A. laidlawii antibiotics was found sensitive to tetracyclines, penicillins, macrolide antibacterial antibiotics, lincomycin, mikamycins A plus B, and neomycins. The mechanisms of action of the A. laidlawii antibiotics appears to involve inhibition of protein synthesis. About 10 percent of the Mycoplasma cultures examined produce some sort of antibiotic activity when grown under appropriate conditions in appropriate media.